This is a follow up to D88792 which found an issue in a call to PExpectTest's
expect function that allows passing a string to the `substrs` parameter. However
this issue was found by just grepping and TestPExpect's expect function is still
accepting a single string as a value to `substrs`.
This patch adds the same sanity check that D88792 added to the PExpectTest's
implementation of `expect` and also adds a small test for it.
Reviewed By: kastiglione, JDevlieghere
Differential Revision: https://reviews.llvm.org/D89302
